Impedance requirements for frequency counters in circuits

[mathlete]

I'm trying to figure out something about frequency counters. When you hook up one up to a circuit, what property of the counter is necessary so it doesn't affect the measurement of the circuit - for example, an ammeter has to have a very low resistance, etc.

edit: I really don't see what this has to do with electrical engineering, I guess all question's pertaining to Ohm's law are not physics now?

 

[Delta]

How is it being used?

Is it a frequency counter IC?, A RPM counter?, a electromechanical lap counter?

 

[faust9]

Frequency counters should have high(ideally infinite) input impedence to prevent loading the circuit--Just like an ohmeter.

In practice you'll find thay(like ohmeters) have 1M to 10M of impedence(give or take a little).

Here's a simple one if you want to look over the schematics: http://www.piclist.com/techref/piclist/weedfreq.htm

 

[waht]

re

Some of the Agilent (Hewlett Packard) counters can switch between a 50 Ohm and a high impedance input. But those cost big bucks.

 

[pack_rat2]

The main source of error with counters is that they can trigger on noise. But this isn't something that affects the circuit. Careful adjustment of the trigger or sensitivity control can improve accuracy.